The present application teaches an apparatus and method for applying an active matrix to the Vacuum Fluorescent Display (xe2x80x9cVFDxe2x80x9d) in order to maximize the brightness of the VFD. This invention teaches how to integrate an active matrix to the VFD using either single crystal silicon chips or thin film transistor (xe2x80x9cTFTxe2x80x9d) techniques for the active matrix, and generally relates to the areas of flat panel displays.
The Vacuum Fluorescent Display (xe2x80x9cVFDxe2x80x9d) is a flat panel display that has been manufactured in Japan and Russia for the last two decades. The VFD has found a marketplace as a messaging display for equipment such as clocks, radios, tape players and CDs in automobiles. It is also found on appliances such as microwave ovens. The VFD is viewed by the industry as a very bright and reliable display for low-resolution alphanumeric and icon displays. It has never found use as a high-resolution graphics display that could be used in the computer monitor, or television markets. The reason this has not occurred is that the high-resolution displays must support animated images, and VFDs presently cannot support such animation.
In order to produce animation the display must be refreshed at some frame rate that is fast enough that the image does not appear to flicker to the human eye. This minimum frame rate is around sixty frames per second. In such displays the frame rate is usually selected at around 75 frames per second so that the frame rate does not coincide with the 60 cycles per second of the alternating current electrical power source.
Cathode ray tubes (xe2x80x9cCRTsxe2x80x9d) are beam-driven displays. In CRTs the frame is painted pixel-by-pixel by sweeping a beam of electrons in a raster scan from side-to-side and down the frame until the complete image is formed. In such displays the electron beam is only momentarily on each phosphor dot (pixel), once for each frame. The human eye""s response is too slow to catch the beam movement and interprets the response as a steady lighted dot, although in reality it is flickering at 60 or 70 times per second. Instead of a flicker the eye sees a low brightness.
Due to the short dwell time of the beam on a particular pixel, the light from the phosphor of the pixel is highly limited. To compensate for the short dwell time the beam power is boosted to extremely high powers and voltages (30,000 volts for the color TV). If the television beam were to remain fixed on the phosphor dot, then that pixel would be extremely bright for a short time and then burn out.
Today all flat displays, including VFDs, are matrix driven devices as opposed to beam driven devices. Matrix driven means that driving the image is obtained by activating columns and rows. The point where a column and row meet defines a pixel. Present matrix driven displays are commonly line-driven as well, as opposed to either raster-scanned (CRTs) or matrix displays that are individually addressable pixel-by-pixel. This means that a total line of the display is enabled together by a single line driver and then image data for the line is fed in parallel to all the columns. The result of this is that the dwell time of the electrons on the phosphor is about a thousand times longer than it is for a raster-scanned display. This means that the electron power can be greatly reduced so that a line scanned VFD need only have 10 to 50 volts to energize the electrons stimulating the phosphors.
The brightness of the line-driven display is impacted not by the number of dots or pixels in a line, but by the total number of lines in the display, as the line may be activated only for the length of time of the given line is active during a particular frame. Hence, increasing the number of lines decreases the amount of time that each line is active, and hence diminishes the brightness of the line. The high brightness of previous VFDs is due to the fact that the matrix in a messaging display has only a few lines (from 1 to 10). The more lines the display has, the dimmer is the image for a particular voltage. This means that a high-resolution display with 500 lines is too dim, and that more brightness has to be attained by turning up the voltage.
However, high voltage displays require that the driver system must be able to handle voltages in the 200 and 300-volt range to obtain the brightness of a line-driven VFD. This causes the driver system to be prohibitively expensive, and therefore not economical. The matrix line scan cannot produce an economically viable high-resolution display.
One solution to this problem is to turn the phosphor pixels on for the total length of the frame. This can be accomplished using a transistor circuit to drive each individual pixel. This was implemented by Peter Brody at Westinghouse in the early 1970s and is called the active matrix (xe2x80x9cAMxe2x80x9d). Today liquid crystal displays employ the active matrix and are called active matrix liquid crystal diodes (xe2x80x9cAMLCDsxe2x80x9d). The active matrix is typically made from amorphous silicon, or poly-silicon.
In 1981, the concept of an active matrix vacuum fluorescent display (xe2x80x9cAMVFDxe2x80x9d) was published by Sahiro Uemura and Kentaro Kiyozumi, engineers working for Ise Electronics, Japan, in the Transactions on Electron Devices, Vol. Ed-28, No. 6, June 1981. In that paper they discussed a pixel memory system consisting of two p-channel transistors and a capacitor in a monolithic integrated circuit silicon chip. This enabled the display to operate at 100 percent duty factor with a 60-Hz refresh rate. The results were, xe2x80x9cin the enhancement of phosphor brightness up to 4000 to 5000 fL at Vp=30 V.xe2x80x9d Having a display with such brightness potential allows it to be used as a projection system, or the filament temperature may be significantly reduced for a substantial power saving, or high filtration can be added to make a daylight-readable display.
Nine years later in the papers for the 1st International CdSe (Cadmium Selenide) Workshop, 1990, a paper presented by Shimojo, Okada and Kamogawa of Ise Electronics, Japan discussed an AMVFD that utilized an active matrix using thin film transistors (xe2x80x9cTFTsxe2x80x9d) fabricated with cadmium selenide for the thin film semiconductor. In Japan, cadmium selenide is considered to be very poisonous and therefore, Ise dropped the use of cadmium selenide shortly thereafter in favor of single crystal silicon chips.
The semiconductor circuits used in AMVFDs are fabricated utilizing CMOS (Complementary Metal Oxide Semiconductor) technology. The CMOS circuit is constructed with an insulating layer of glass deposited over the circuitry and interconnects, with an aluminum anode pad deposited over the glass and connected to the drain of a power FET (Field Effect Transistor) under it. Phosphor of the proper formulation is then deposited on the aluminum pads. These chips were then mounted on the base glass of the vacuum envelope with filament wires strung over the phosphors. The image is viewed through the filaments, but they are so thin that they are not seen at the viewing distance.
Prototype displays were tested and were found to have four times the brightness of commercially available VFDs. The difficulty with the silicon chip system is that each chip has to be carefully aligned with the chip on either side of it and with the chip over and under it. Also, since the chips cannot be abutted up against each other (because chips need area around the circuitry to be xe2x80x9cdicedxe2x80x9d and for power lines) and because each chip is not exactly like the next chip, some room has to be afforded between each chip. This reduces the amount of phosphor surface area and also the number of pixels per linear unit, because the space between chips must also be the same as the space between pixels on the chip otherwise the display will not be uniform, but will have lines crisscrossing it corresponding to the cracks between the chips. Thus, high-density graphics displays are not possible using the silicon chip technique.
Another problem with present AMVFD displays is that they have no gray scale capability beyond a simple binary (single bit) display. In a binary system the pixels are turned on or off with no intermediate levels of shading. In a true grayscale system there are a number of intermediate levels of shading, either with continuous shading for an analog driven system or a number of discrete levels determined by the number of bits for a binary driven system. The simple on/off binary arrangement is inadequate for providing color or high-resolution displays.
Ise was not able to follow up on their AMVFD work because the use of monolithic silicon chips for the active matrix was economically prohibitive. Today Ise markets small silicon chip-driven AMVFD of low resolution for alphanumeric displays and which is the mainstay of their VFD business.
Other companies have attempted to create active matrix displays, also with limited success. One example is the active matrix display taught in Curtin et al., U.S. Pat. No. 5,686,790 and assigned to Candescent Technologies Corporation. In this display a substrate contains a matrix of holes containing emissive electron sources known as Spindt cathodes. A glass pane patterned with phosphors is spaced above the substrate by a collar that surrounds the display area and spaces the glass pane above the electron sources, and a vacuum is created between the glass pane and the substrate. Electrons are projected from the Spindt cathodes away from the substrate onto the phosphor of the glass pane to produce the image. In one embodiment the rear pane forms an envelope that encloses the substrate. Control circuitry is placed external to the display and electrically connected to the cathodes by means of traces through the substrate.
This display has several problems. Because the phosphor pixels are located on the glass pane and a vacuum is created, the glass bows inward, and a non-uniform emission pattern is created on the pane. This requires the use of internal spacers between the substrate and display glass pane, which dramatically complicates construction of the display and degrades the quality of the image. In addition, focusing the electron streams is difficult because all of the control circuitry is located within the substrate and hence close to the electron source. Small angular discrepancies at the source lead to significant linear discrepancies at the glass pane.
As a result of these and other problems, the Curtin et al. display cannot be manufactured to provide the performance needed for high-resolution full-motion displays at an affordable cost.
One method of overcoming the control issues of the Curtin et al. display is to replace the arrangement of a cathode electron source spaced from a phosphor pixel in a vacuum with a sandwich of cathode and anode strips with an electroluminescent (EL) material disposed between them. Two examples of such inventions are Khormaei et al., U.S. Pat. No. 5,652,600, assigned to Planar Systems, Inc. and Swirbel et al., U.S. Pat. No. 6,091,194, assigned to Motorola, Inc. These displays have suffered from difficulty in creating EL materials that can provide a full range of color. Although significant progress has been made with such displays, present materials and manufacturing techniques do not allow full-motion, high-resolution color displays to be manufactured at an affordable cost.
Although the AMVFD has been invented and has demonstrated it can solve the brightness and power problem associated with passive matrix VFDs, no one has been able to capitalize on it because of an inability to produce a manufacturable approach to achieving it.
The present invention teaches an active matrix vacuum fluorescent display device. The display has an envelope for enclosing a space containing a vacuum. The envelope further comprises a first pane of a transparent material, preferably glass, and a second pane substantially parallel to the first pane, enclosing a vacuum space. A substrate panel having first and second parallel sides is disposed between the first and second panes of the envelope, within the enclosed vacuum space. The substrate panel and the first pane are spaced from one another, with the first side of the substrate panel being closer to the first pane of the envelope than is the second side of the substrate panel. A plurality of conductive anode pads are disposed on the first side of the substrate panel, those conductive anode pads being covered with light-emissive phosphor coatings.
Driver circuitry for selectively activating individual conductive anode pads is disposed on the second side of the substrate panel. A plurality of conductive vias connect the conductive anode pads to the driver circuitry, with a unique via connecting each separate conductive anode pad to the driver circuitry.
Finally, an electron source is disposed between the substrate panel and the first pane of the envelope. It is spaced from the first pane such that a vacuum space is maintained between the electron source and the conductive anode pads. In a presently preferred embodiment the electron source comprises a plurality of thermionic filaments.
The substrate may be disposed entirely within the vacuum envelope, or may be integrated into the envelope by use of a first and second side collar, which seal the space between the substrate and the first and second panes, respectively.
In an alternative embodiment the substrate panel replaces the second pane and comprises the rear plane of the display.